Nonwoven fabrics and fabric laminates are widely used in a variety of applications, for example, as components of absorbent products such as disposable diapers, adult incontinence pads, and sanitary napkins; in medical applications such as surgical gowns, surgical drapes, sterilization wraps, and surgical face masks; and in other numerous applications such as disposable wipes, industrial garments, house wrap, carpets and filtration media. For example, nonwoven barrier fabrics have been developed which impede the passage of bacteria and other contaminants and which are used for disposable medical fabrics, such as sterilization wraps for surgical and other health care related instruments, surgical drapes, disposable gowns and the like.
Barrier fabrics can be formed by sandwiching an inner fibrous web of thermoplastic meltblown microfibers between two outer nonwoven webs of substantially continuous thermoplastic spunbonded filaments. The fibrous meltblown web provides a barrier impervious to bacteria or other contaminants in the composite nonwoven fabric, and the spunbonded webs provide abrasion resistance and integrity to the laminate. Examples of such trilaminate nonwoven fabrics are described in U.S. Pat. No. 4,041,203 and U.S. Pat. No. 4,863,785.
Current industry standards require that laminate fabrics used for barrier purposes provide a predetermined level of protection against penetration of the fabric by air borne contaminants. The level of barrier protection required can depend upon the particular end use application of the fabric. Many laminate fabrics currently available cannot meet all of the requirements for a particular end use application.
In addition, conventional trilaminate barrier fabrics can also be limited with regard to the types of sterilization procedures which can be used therewith. For some applications, it is desired that the fabric or garment be sterilized in the final stages of manufacture by exposure to gamma radiation. For example, the fabric or garment may first be sealed in a protective package, and then exposed to gamma radiation to sterilize the package and its contents.
However, sterilization by gamma irradiation has been found to be unsuitable for many of the known medical barrier fabrics. Some of the polymers conventionally used in such medical barrier fabrics, such as conventional grades of polypropylene for example, are especially sensitive to degradation by gamma irradiation. Fabrics produced from such polymers tend to lose strength over time, becoming brittle as a result of the gamma irradiation. Also, the instability of the polymers to the irradiation results in the generation of distasteful odors in the product which are unacceptable to the consumer.
Various attempts have been made to overcome these limitations. For example, efforts have been made to render the polypropylene polymers more stable to gamma irradiation, such as by incorporating certain additives in the polymer to reduce the amount of degradation. For example, U.S. Pat. No. 4,822,666 describes a radiation stabilized polypropylene fabric in which a long-chain aliphatic ester is added to the polymer. U.S. Pat. No. 5,041,483 discloses incorporating a rosin ester into the polypropylene to stabilize the polymer and reduce the tendency toward odor generation after gamma irradiation. However, the use of such additives adds expense to the manufacturing process. Further, polypropylene is difficult to render gamma-stable even with the use of additives or stabilizers.
Other polymers have good stability upon exposure to gamma irradiation, such as polyethylene. However, there are problems associated with the use of polyethylene to form nonwoven webs, specifically as the meltblown component of a trilaminate fabric. For example, polyethylene generally exhibits poor spinnability, particularly at high spinning speeds. Yet high spinning speeds are highly desirable for successful commercial production of polyethylene fibers. Further, it is difficult to produce fine denier fibers at commercially feasible spinning speeds. This is especially true as fiber size decreases to the 1 to 50 micron range useful for imparting to a fabric the degree of barrier protection required by industry standards.